Electron microscope



Oct. 13, 1953 VERHOEFF I 2,655,601

ELECTRON MICROSCOPE Filed March 24, 1950 INVENTOR. ADRIANUS VERHOEFF AGENT Patented Oct. 13, 1953 ii afi s Vei 9 .i 1 v! emesis, "signer 'to Hartford National Bim'k ma com any, H'aitford, Conn as trustee Aiplici iioh March 24, 1950, fs'eriaiiim. 51,556

' 5 claims.

This invention relates to electron microscopes, in which the object traversed by the electron rays and a limiting diaphragm are arranged close to each other in the path or rays within the apparatus. This arrangement/is frequently used in order to intercept marginal rays, \r1 hioh are produced by fdijsperslidn "in the fo'bject and whi give rise to diffusion or the beam, thus detracting from the "qualityof the image,

'Ihe object is, es a rule, secured in a homer, introduoedinto the v'acuous spate, here being taken to ensure that the objeot is located at the centr'e of the beam after the introduction of the holder. The correct orientation of the Object relatively "to the beam "often requires, that "the object should be slig'htly'displaceable in the plane at right angles to the beam of rays. This is required more particularly if a *narro'wfbeam is separatedfrom the rays passing through the object by means of a diaphragm arranged 'behind the tibject, 'siiice the aperture in the di- "aphr'ag'r n is smaller than the io'ii-s'eetion of the beam. In tliiscase theob'jct must be'di placeable relatively to the aperture in the diaphragm, jinforder, to find L'the 'ebrrect adjustment essentia'lfo'r the reproduotio'no'f the image. g

The invention relates to aconstructionwhioh provides this possibility and furthermbie relates Ito the "device for displacing the object and to the air-'tightseals "betweenthe displaceabl'e parts ancl'the fixed parts'of the microscope, I

According to the invention, 'th'e'ob'jeot is arranged in a plate-shaped disc having a'ceiitral aperture to allow passage'of the beam, this disc being displaceable in the plane at right angles to the path of rays and being'ar'ranged between twosubsequent parts of the wall of the microscope surrounding the beam chann'eL'an airtight "seal being provided between each-o'f'thes'e parts and the disc. r

The object requires only small-displacements.

:The largest displacement from the centre of the electron beam is about lmillimeterfbut in'most cases a displacementof some few'tenths of a millimeter will be sufficient. This small displacement imposes severe requirements'on the moving mechanism, for example, the absence of any amount of play.

This is ensured by providing the disc comprising the object with two slightly flexible metal fr'o'ds "extending to the exterior and radially ar- 'r'anged'at an angle of 90. The ends of the rod jc'o o'perate with a lever arrangement, with the use offjwhich push andp'ull forces injth'e 'direc- 'ti'on' of length 'of the rods'can be exertedto' disthe Netherlands Aid-i1 9, 1949 p ec the disc. In order to avoid any may between the adjusting mechanism and the lever frigid-1y 'secured tb the "disc, the free of the lever is pih'ohefd "between a "rod sl-id'alb'le in the dirotion of its length and c'o bprajting "with an adjlusting frieii'lher arranged in the proximity of the screen of the i'fiiorosdpe and "a resiliently rranges ri'xe'rifbei' 'whieh exerts a fofc'e the :lefv'ejr adi-rectiGn against the bush rod. The adjusting member brings abbut a displacement o f tl'fe push "rod and *thus the desired displacenens the object relatively to the oe'ritre bf wh n t v,

The air-"tight "seal of the dis'c relatively to the a'dj'aoent s'tati dnary part bf th e microscope, Whieh ciir'iipr'ise "the beamchannel, may be married out in the following manner. l'he disc -'is 'pidvided either side of the "object Chamber "with central reesss into whioh e'iitend conical ends "of the stationary "parts of fthe microscope. The seal is "ensured with the use of toroidal rubber rings ofcjirdiflm section which "are pressed "into the aii'gles between the odriical surfaces of t eitationafy parts ant the 'siirfaces o'njeither is ide of ftlie dish, "the rings being pressed diie to the "diifeieho'e in pressure externally and interfisny or {the mier s ope. pressure "may be increases "by providing "a diameter for the rings similar than "the crosnse ction as the embratl'ed b its, :So "that the "rings are vi idehe dpn "being displaced over the eonical sur'fa'qe's. The resulting stress in the material urges the rings on the surface "of the disqso "thait' sealing is ens'uredeven the absence are pressure dilferame.

"In ldr de'r that the invention may The "readily carries iht'o street, an example will ow behejscribe d 'in "detail wit -reference "to the "ac'c'ofnpanyingfdi'awingsin which- Fig. 11 is "a "longitudinal sections-1 view-oi'imt bi an "electron microscope; V "'Fig. 1a shows the object holder assemblytof Fig. 1 in :greate'r "detail; and

The left hand halffo'f Fig.1 is afs'e'tional View takehfon'th'e line ABa'n'd therighthand half seotional vievwtakenbn thelineBCbfFig. 2. "electron foussifngsystem ebmprising a pair magnetic fields-tor iocussing;le s s-arranged an :eith'erjs'ide tr thepreparatio'n' '(notshe'wn'). Each ooihsurrou'nds ?a jentral' polefpiece I3, 4, respec- "tively piovidetl with a passage "for the "'elcing this beam channel leave some intermediate spacing, in which the object is arranged, an image of which is to be formed on a screen (not shown) with the use of the electron beam. The object I is secured to the end 42 of a holder, which is constituted by a thin rod 5. The latter is secured to a knob 8 provided on the outside of the microscope and which enables the object holder to be removed for replacement of the object. It is evident from Fig. 2 that, on removing the object holder, the channel accommodating the holder is closed, in that a valve head 9 is displaced due to pressure of a spring l until it closes the aperture of the channel. The object holder 6 has a small aperture H, on which the object is clamped tight by a resilient piece of metal 12. When the object holder is arranged in the apparatus the aperture of the holder corresponds to the centre of the beam of rays.

Below the object and in the proximity thereof, is a beam diaphragm I3 constituted by a metal plate having an aperture 43 which is smaller than the aperture of the object holder. The diaphragm is clamped against the wall I! of the microscope, with the use of the holder l4, two screws l and a nut l6, secured to the holder. The securing is such that, with the use of the screws l5 and the nut 16, the diaphragm aperture can be brought exactly at the centre of the beam. The beam diaphragm proper is secured to a member provided in the holder and connected with a knob l8, so that the diaphragm can be removed without any change in the adjustment of the holder. The holder extends through a radial aperture in the disc and issecured to the disc by way of a flexible coupling, which is also air-tight.

It is quite possible that it is required to examine a part of the object other than that atthe centre of the beam in front of the diaphragm aperture. This could be effected by displacing the diaphragm, but this also results in a displacement of the image of the object on the screen. Therefore, it will be preferred to displace the object. For this purpose the object holder is arranged in a flat disc IQ of circular section, which is supported in a groove 20 provided in the wall I! of the microscope, the diameter of the groove slightly exceeding that of the disc, so that the latter has a certain amount of play in any direction in the plane at right angles to the axis of the beam.

The displacement of the object is, as a rule, limited to some few tenths of a millimeter and must therefore be effected with great accuracy. The adjusting mechanism is shown in the righthand half of Fig. 1. This figure is a sectional view taken on the line BC of Fig. 2. Connected to the disc [9 are two radial rods 2| and 22 arranged at an angle of 90 and clamped in metal blocks 23 and 24. Fig. 1 shows that block 23 is clamped on a lever 25 with the use of a screw 26. The lever 25 has a centre of rotation in the shaft 21, to which it is connected with the use of a clamping screw 28. This results in a lever arrangement, of which the short arm is secured to the disc I9 and the long arm is clamped between a push rod 29 and a member 30, which is slidable in a sleeve 3| and urged by a spring 32 to the exterior and against the lever. The clamping of the lever between the push rod 29 and the member 30 prevents play at the points of application, so that any displacement of the push rod is transmitted directly to the lever arm. This displacement is effected by means of an adjustable screw bolt 33, which is provided with a knob 34 to facilitate the control. Any play in the thread of the adjusting screw 33 is also eliminated by the pressure of the spring 32. Adjustment may be effected with the required accuracy by using micrometer thread on the adjusting screw 33. In this construction the adjusting screw may be provided in the proximity of the screen of the microscope, if push rods of sufiicient length are used, so that any variation in adjustment can immediately be checked on the screen. The supports 35 and 36 for the various parts of the adjusting mechanisms are secured to the wall ll of the microscope.

Provision must be made to ensure that air cannot penetrate into the beam channel upon movement of the disc, so that special care must be taken in sealing the disc and the stationary parts of the microscope embracing the discharge path. In the construction shown, the beam channel is located in the parts 3 and 4 and scaling is effected between these parts and the disc I9. For this purpose use is made of rubber rings 31 and 38 of circular section. The disc I! is provided, on either side of the object space, with central recesses 39 and 40 into which extend the conical ends of the parts 3 and 4. The recesses are slightly wider than the smallest diameter of the conical ends of the parts 3 and 4, so that they do not hamper the movement of the disc. The rubber rings 31 and 38 are urged by the difference in pressure between the inside and outside of the vacuous space into the angle made between the conical surfaces of the parts 3 and 4 and the surfaces of the disc [9. By providing for the rings a diameter smaller than the cross-sections of the parts 3 and 4 at the area of sealing, so that these parts in assembling the microscope are displaced along the conical surfaces and are thus widened, it is ensured that sealing is obtained even in the absence of the pressure difference. Air is thus prevented from leaking in, when the process of exhausting of the tube is initiated.

With a view to assembling the microscope, the wall I! is divided into two parts along the surface 4 I.

What I claim is:

1. In an electron microscope, an object chamber, an electron lens system arranged within said chamber comprising a pair of axially aligned tubular pole-pieces, a pair of axially aligned focussing coils, each of said coils surrounding one of said tubular pole-pieces, a disc interposed between the pole-pieces and having a central aperture therein, the surrounding surface portions of which interfit with corresponding surface portions of the tubular pole-pieces to form therewith a discharge path for an electron beam through said disc, a resilient sealing member connecting the adjoining surface portions of the disc and each pole-piece, means to move the disc in a plane perpendicular to the path of the electron beam, an object holder movable in a channel in the disc transverse to the path of the electron beam, valve closing means for sealing said channel upon withdrawal of the object holder, and a beam diaphragm in the central aperture of the disc between the object holder and one of the pole-pieces.

2. In an electron microscope, an arrangement as defined in claim 1 in which the disc moving means comprises a pair of slightly flexible rods attached at right angles to the disc, and a lever connected to one of said rods for exerting 8. Ion- 5 gitudinal force on the rod thereby displacing the diaphragm in the disc relative to the center of the electron beam.

3. In an electron microscope, an arrangement as defined in claim 2 in which the free end of the lever is clamped between a push-rod and a resilient member exerting pressure on the lever against the push-rod.

4. In an electron microscope, an arrangement as defined in claim 3 in which the push rod is provided with an adjusting member positioned in the proximity of a screen for viewing the electron beam after passage through the object chamber.

5. In an electron microscope, an arrangement as defined in claim 1 in which the adjoining surface portions 01 the disc and the pole-pieces are conical, the disc being recessed about the aperture to receive the pole-piece.

ADRIANUS VERHOEFF.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,220,973 Marton Nov. 12, 1940 2,361,722 Von Ardenne Oct. 31, 1944 

